106 



APPLIED PHYSIOLOGY 



The force with which the fibres contract seems to 

 depend largely upon the pressure in the interior of the 

 heart, or, in other words, on the extent to which it is 

 filled during diastole. If, for example, the heart ' misses 

 a beat,' and in consequence of that the left ventricle 

 becomes overfilled, the next systole is unusually power- 

 ful, and is felt as the c thump ' so often complained of by 

 patients with palpitation. 



Although, as has just been pointed out, the normal 

 contraction starts at the venous ostia, yet it must be 

 remembered that the whole of the heart muscle possesses 

 the power of originating stimuli, and a contraction set 

 up in the ventricles will pass back to the auricles. In 

 cases of disease this abnormal mode of cardiac contrac- 

 tion sometimes occurs. The capacity of transmitting a 

 wave of contraction is spoken of by physiologists as the 

 conductivity of the heart muscle, and we shall see later 

 on that alterations in conductivity explain many of the 

 alterations in cardiac rhythm which can be observed in 

 disease. The ventricles respond to the contraction wave 

 by a simultaneous and co-J^faated shortening of all 

 their fibres not by a peril^^c wave by which the 

 ventricular systole is producecBp These fibres are partly 

 circular and partly longitudinal in arrangement. The 



1906, ii. 359). The nmsdl*"fibres which make up its ramifica- 

 tions are of the Purkinje type, and possess a lower degree of con- 

 ductivity than the more highly developed fibres of the heart wall. 

 If the bundle is destro^d by disease, the propagation of the wave 

 of contraction is prevented, and a condition of 'heart-block' 

 results. In such a case the ventricles take on an independent 

 action, and contract <fcythmically at a rate of 28 to 36 beats per 

 minute. % 



